Conventional lithographic printing techniques cannot accommodate true high-speed variable data printing processes in which images to be printed change from impression to impression, for example, as enabled by digital printing systems. The lithography process is often relied upon, however, because it provides very high quality printing due to the quality and color gamut of the inks used. Lithographic inks are also less expensive than other inks, toners, and many other types of printing or marking materials.
Ink-based digital printing as discussed in this disclosure uses a variable data digital lithography printing system, or digital offset printing system. A “variable data digital lithography system” is an image forming system that is configured for lithographic printing using lithographic inks and based on digital image data, which may be variable from one image to the next. “Variable data lithography printing,” or “digital ink-based printing,” or “digital offset printing” are terms that may be generally interchangeably employed to refer to the processes of lithographic printing of variable image data for producing images on a wide latitude of image receiving media substrates, the images being changeable with each subsequent rendering of an image on a substrate in an image forming process.
For example, a digital offset printing process may include transferring radiation-curable ink onto a portion of a fluorosilicone-containing imaging member surface that has been selectively coated with a dampening fluid layer according to variable image data. The ink is then cured and transferred from the printing plate to a substrate such as paper, plastic, or metal on which an image is being printed. The same portion of the imaging plate may be cleaned and used to make a succeeding image that is different than the preceding image, based on the variable image data. Ink-based digital printing systems are variable data lithography systems configured for digital lithographic printing that may include an imaging member having a reimageable surface layer, such as a silicone-containing surface layer.
Systems may include a dampening fluid metering system for applying dampening fluid to the reimageable surface layer, and an imaging system for laser-patterning the layer of dampening fluid according to image data. The dampening fluid layer is patterned by the imaging system to form a dampening fluid pattern on a surface of the imaging member based on variable data. The imaging member is then inked to form an ink image based on the dampening fluid pattern. The ink image may be partially cured, and is transferred to a printable medium, and the imaged surface of the imaging member from which the ink image is transferred is cleaned for forming a further image that may be different than the initial image, or based on different image data than the image data used to form the first image. Such systems are disclosed in U.S. Publication No. US 2012/0103212A1 (“212 Publication”), entitled “Variable Data Lithography System,” filed on Apr. 27, 2011, by Timothy Stowe et al., which is commonly assigned.
Variable data lithographic printing system and process designs must overcome substantial technical challenges to enable high quality, high speed printing. For example, digital architecture printing systems for printing with lithographic inks impose stringent requirements on subsystem materials, such as the surface of the imaging plate, ink used for developing an ink image, and dampening fluid or fountain.
Fountain solutions or dampening fluids, such as octamethylcyclotetrasiloxane “D4” or cyclopentasiloxane “D5” may be applied to the reimageable surface of the imaging member that may be in the form of a printing plate or an intermediate transfer blanket. Subsequently, the applied layer of dampening fluid is image-wise vaporized according to image data to form a latent image in the dampening fluid layer, which may be about 0.5 microns in thickness, for example. During the laser imaging (vaporization) process, the base marking material layer is deposited in a uniform layer, and may spread across the background region, allowing subsequently applied ink to selectively adhere to the image regions. A background region may include D4 between the reimageable surface or plate and the deposited ink. A thickness of the dampening fluid layer may be preferably around 0.2 microns, or more broadly in a range of about 0.05 and about 0.5 microns.
The laser used to generate the latent image in the dampening fluid layer creates a localized high temperature region that is at about the boiling point of the dampening fluid, e.g., about 175° C. Accordingly, during the imaging process, large temperature gradients are formed on the reimageable surface of the imaging member in the imaged areas. The surface temperature rapidly decreases to ambient temperature away from the imaged areas or imaging zones, i.e., the portion of the reimageable surface of the imaging member on which the imaging (laser imaging) takes place.
Due to a motion of the imaging member surface during printing, dampening fluid vapor has been found to migrate over cooler regions of the imaging member surface, allowing the vapor to re-condense on the imaging surface. If re-condensation occurs over an imaged region of the imaging member surface, streaks may appear in the printed image. Dampening fluid vapor must be removed before it re-condenses on the imaging member surface. Related art dampening fluid vacuum recovery systems are limited to low process speeds, for example, less than 500 mm/s.
A consistent thickness of a dampening fluid layer formed on the reimageable surface of an imaging member, and inhibiting a variability of the thickness of the disposed layer over the reimageable surface of the imaging member, or over the plate surface, is critical to effective high-quality image printing operations. To obtain a uniform dampening fluid layer thickness, reimageable surface or plate surface conditions must be satisfied. For example, under suitable conditions, a reimageable surface of the imaging member may be characterized by uniform temperature, and concentration of the dampening fluid may be uniform, and a mixture velocity tangential to the reimageable surface of the imaging member or imaging plate motion may be uniform.